Bandpass filters with improved upper stopband performance using dual-behaviour resonators

Abstract

Two novel bandpass filters with improved upper stopband using a pair of dual-behaviour resonators (DBRs) are presented in this paper. By utilising coupled lines and open stubs, the second harmonic can be eliminated easily. Extended DBRs with different lengths are used to obtain more transmission zeros to suppress the third and fourth harmonic. 15-dB rejection level in the upper stopband from 1.3f0 to 2.72f0 and 5.2f0 (f0 is the centre frequency of the passband) can be obtained for the two proposed bandpass filters, respectively. Two planar bandpass filters (3-dB fractional bandwidths 9.3%, 8.9%) are designed and fabricated. Good agreements are presented for the theoretical and measured results, which show good in-band filtering performances and high selectivity.